The method of measuring blood pressure that is most universally known consists in compressing the arm of a person with an inflatable cushion until the blood is interrupted, after which the pressure is progressively decreased and the re-establishment of the blood circulation is detected acoustically by means of the "Korotkoff noises" which are produced when the pressure decreases from the systolic pressure to the diastolic pressure. The "Korotkoff noises" are detected by means of a stethoscope place between the inflatable cushion and the skin.
There has already been proposed an automation of this process for wider public use by incorporating a microphone into the inflatable cushion and by connecting it to an electronic circuit for automatically determining the systolic and diastolic pressures. A drawback of such arrangements arises mainly from the fact that it is not easy to wrap by oneself an inflatable cushion around the arm and that it is necessary to remove any clothing covering the arm. Although such constraints may be unimportant within the frame of the medical profession, they constitute an impediment to wide public utilization, especially when the pressure measurement is to be made on a person's own body.
French Pat. No. 1,334,572 describes an apparatus for measuring blood pressure by the detection of light-absorption variations through a portion of the ear lobe subjected to a variable pressure. Thus, the application of such pressure drives the blood of this portion from the ear lobe and the detecting device will measure the volume of remaining blood. The measured value, of course, varies with the same frequency as the heartbeat. For drawing a curve of the transparency variations, it is necessary to measure the maximum values and to calculate the curve from these values, after which the systolic and diastolic pressures are deduced from some specific values of this curve on the basis of particular relations existing between these values and the systolic and diastolic pressures.
The calculation of the curve of pressure variations requires the use of a relatively complex electronic circuit which results in a rather expensive system not suitable for making an apparatus intended for wide public use. Moverover, the transparency measurement which is contemplated in the French patent can only be made on the ear lobe which is not an ideal measuring spot for an apparatus designed for use by the general public and not reserved exclusively to doctors.
French Pat. No. 2,052,617 and U.S. Pat. No. 3,412,729 concern oxymeters from which the blood pressure can be obtained indirectly. This measurement is based on the absorption of infrared light by tissues which varies in inverse proportion to the amount of blood, the latter varying in turn according to the blood pressure. The signal given by this measurement is alternating which makes the determination of the diastolic and systolic pressures relatively difficult to perform. Such devices are generally intended for hospitals and are not suited for being sold outside the specialists' field .
U.S. Pat. No. 3,698,382 concerns a device for driving the blood from the area underneath a portion of skin by pressure application and for measuring the blood-return rate in such area which is a function of the blood pressure. Such a device is, however, not adapted for measuring systolic and diastolic pressures.
It has already been proposed to measure the blood pressure from a finger. In such a case, the blood stream can be detected optically by means of a photoplethysmograph which measures the light transmission through the skin varying in accordance with the heartbeat. Consequently, when the blood circulation is stopped, the signal from the photoplethysmograph is essentially constant; it becomes again variable when the circulation is re-established.
This process enables a general measurement of the systolic pressure, with extremely questionable reliability. A measurement of diastolic pressure by this process has no practical significance since large variations occur from one measurement to the next.
The difficulties result from several factors, including the pressure difference between the arm and the fingers where the arteries have a much smaller diameter. Even if the pressure is measured correctly, the result will be different from the value obtained on the arm which constitutes a unviersal standard. Moveover, the pressure measured in this manner can vary under the influence of other factors so that it is difficult to set up comparisons between several measurements on the same person, owning to the vaso-constrictive automatic control of the quantity of blood that circulates in the extremities (hands and feet) as a function of the energy requirements of other parts of the body. Thus, the amount of blood in the fingers is smaller during digestion or when the weather is cold, which is an additional factor that may alter the pressure difference between the arm and the fingers and may also cause discrepancies between two measurements done on a finger when the intervening time is significant. Thus, not only is the finger measurement not comparable to the arm measurement, but two successive finger measurements done under different conditions are frequently divergent.